In optical waveguide systems, there is a need for a coupling channel waveguide whose width continuously varies from one dimension to another over a relatively short length of the waveguide. This need may arise, for example, when the waveguide is used to couple a single mode optical fiber whose core has one dimension to an optical waveguide whose channel width has a different dimension. To couple efficiently, the waveguide used for coupling should be relatively lossless and remain a single mode waveguide despite the change in its channel width between the two dimensions. The latter consideration requires that the index of refraction along the channel vary inversely with the change in its geometry. These factors pose problems.
In particular, if the desired change in channel width is achieved simply by forming, by the normal photolithographic techniques, a channel whose width tapers gradually between the two dimensions needed, the index of refraction of the channel guide tends to remain uniform along the length of the tapered region because the concentration of the impurity added to form the index of refraction change in the channel tends to be uniform along such length. As a consequence, because the width of the channel varies along such length while the index of refraction remains uniform along the length, the modal properties along the region of taper vary. What is needed to maintain the modal properties essentially constant along the length where the channel width varies is a compensating change in the index of refraction along such length.
The problem is especially critical with waveguides that use a large index of refraction change between the channel and its substrate to achieve tight confinement of energy in the channel. The large index change results in a large modal mismatch between the relatively narrow single mode channel waveguides useful in integrated circuit devices and the typically wider optical fibers that are often coupled to such channel waveguides.